Method for producing foamed plastic hollow bodies

ABSTRACT

In the method of producing a hollow foamed plastic body from polyethylene or polypropylene by means of an extrusion blow-molding process using a single-screw extruder and a blow-molding tool, the plastic material is foamed by means of a chemical blowing agent, and a pre-molded tube blank formed from the foamed plastic material is blow-molded in the blow-molding tool. The blow-molding tool has a tool surface temperature of at least 35° C. over at least one portion of the surface. A blowing agent that decomposes endothermically and provides a nucleation effect is included in a blowing-agent master batch based on high density polyethylene. The master batch has an average grain size of about 1 mm and the pre-molded tube blank is blow molded at a pressure of from 0.5 to 2.0 bar, or the pre-molded tube blank is blow-molded in an interval blow-molding process with a blow-molding pressure of from 3 to 10 bar, preferably 8 to 10 bar, in a first time slot and a blow-molding pressure of from 0.5 to 1.0 bar in a second time slot.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a method of producing a hollow foamedplastic body made of polyethylene or polypropylene in an extrusion,blow-molding process with a single-screw extruder and with ablow-molding tool, in which the plastic material is foamed by means of achemical blowing agent and a pre-molded tube blank formed from thefoamed plastic material is blow-molded in the blow-molding tool.

In the course of attempts to reduce material outlay for hollow plasticbodies, in particular in packaging, it is known to produce packagingcontainers (bottles) from foamed plastic material. Packaging containersof this type are frequently used in areas of application in which asmooth exterior surface is an essential functional feature, so as topresent an attractive appearance of the product. However, the foamedhollow bodies produced in extrusion blow-molding have an irregularbulging of the exterior surface caused by gas bubbles in the foam. Thedistribution of the gas bubbles in the wall of the foamed hollow bodiesis also frequently not sufficiently homogeneous, which results inirregular gas cavities and which affects in turn the exterior surface aswell as the mechanical properties of the wall. These effects areparticularly serious where the wall thicknesses of the hollow bodies arein the range of less than 1.5 mm. In order to avoid this drawback in thecase of extrusion blow-molded foamed hollow bodies, a compact, smoothouter skin for example is extruded on in the coextrusion process. Othermethods, such as for example in accordance with DE 1504359-A, suggestproducing a compact outer skin by rapid cooling of the extruded tube onthe outside thereof before the blow-molding process. In this case, anattempt is made at the same time to regulate an uncontrolled foamexpansion on the tube.

In a further publication PCT/EP96/05485 it is proposed to produce asmooth outer skin by encasing a container with a smooth containerdecoration.

All the steps and methods described have the drawback that specialapplications have to be performed with respect to the smooth outer skin,which increase costs in such a way that a substantial proportion of thecost advantage is offset by the foamed hollow body.

In a further publication DE 19525198-A a method is described which isintended to allow an extrusion blow-molded foamed hollow body to beproduced whilst maintaining specific parameters. For this purpose, afoaming agent is admixed with a commercially available PE granulate.This usually takes place as a batch granulate on the basis of PE with agrain size of from 3 to 5 mm or as a powder which is admixed with the PEgranulate. In addition, the plastics-material melt is heated to from 20to 30° above the melting temperature of the compact plastics materialand a pressure of the blowing air of from 0.5 to 2.5 bar is applied. Inthis case, the blow-molding tool is cooled to a temperature of from 7 to15° C. When setting these parameters, which only represent the priorart, although a foamed hollow body can be produced, it does not meet therequirements with respect to the desired properties, in particular thesmooth outer surface and the fine distribution of the gas bubbles in theplastics-material matrix.

SUMMARY OF THE INVENTION

The object of the invention is to provide a method of making a hollowfoamed plastic body of the above-described kind from polyethylene orpolypropylene by an extrusion blow-molding process with a single-screwextruder and a blow-molding tool respectively by which a smooth exteriorsurface and a fine distribution of the gas bubbles is achieved by methodsteps which require practically no costly additional outlay.

This object, and others which will be made more apparent hereinafter, isattained in a method of producing a hollow foamed plastic body made ofpolyethylene or polypropylene in an extrusion, blow-molding process witha single-screw extruder and with a blow-molding tool, in which theplastic material is foamed by means of a chemical blowing agent and apre-molded tube blank formed from the foamed plastic material isblow-molded in the blow-molding tool.

According to the invention the method comprises the steps of:

a) providing a blowing-agent master batch that disintegratesendothermically and provides a nucleation effect during the blow-moldingprocess and has an average grain size of about 1 mm;

b) making a mixture comprising a polyethylene granulate and a portion ofthe blowing-agent master batch, said polyethylene granulate comprisinghigh density polyethylene;

c) extruding the mixture formed in step b) with a single-screw extruderto form a pre-molded tube blank; and

d) blow molding the pre-formed tube blank at a blow-molding pressure ofbetween 0.5 and 2.0 bar with a blow-molding tool having at least oneportion of a tool surface at a tool surface temperature of at least 35°C. to form the hollow foamed plastic body; OR

blow molding the pre-formed tube blank in an interval blow-moldingprocess in a first time slot and in a subsequent second time slot,wherein a blow-molding pressure is between 3 and 10 bar in the firsttime slot and from 0.5 to 1.0 bar in the second time slot and with ablow-molding tool having at least one portion of a tool surface at atool surface temperature of at least 35° C.; whereby the hollow foamedplastic body is formed.

When the method features according to the invention are applied, foamedhollow bodies with the properties described above and with a practicallysmooth exterior surface are surprisingly formed. In this case, noadditional steps or additional devices are required. If one of thecharacterizing features is substantially changed, an immediatedeterioration of the surface occurs, in that it has fine bulges forexample or satisfactory density reduction or foam density is notachieved in the area described or the size and distribution of the foambubbles is irregular.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

For a smooth exterior surface it is particularly important that theblow-molding pressure should be set specifically in the range of between0.5 and 2.0 bar as a function of the shape of the hollow bodies and thewall thickness, in which case shapes with relatively large depressionsand sharp comers/edges require the pre-molded tube blank to be blown outmore vigorously with a blow-molding pressure in the higher pressurerange of up to 2.0 bar.

Hollow bodies or bottle outlines which have to be molded in a specialmanner (acute corners and edges) can no longer be produced with ablow-molding pressure of up to 2 bar, but only by means of a so-calledinterval blow-molding process, a blow-molding pressure of from 3 to 10bar (depending upon the wall thickness) being provided in a first timeslot (preferably from 0.5 to 2.0 seconds) and a blow-molding pressure offrom 0.5 to 1.0 being provided in a second time slot (preferably from 3to 5 seconds).

In this way, for a smooth exterior surface it has been found to beparticularly advantageous to provide a tool-surface temperature of from35 to 40° C., preferably 35° C., as a result of which the gas-pressurepattern is advantageously affected during the formation of gas bubblesfrom the blowing agent.

In a further development the tempering of the surface of theblow-molding tool involves only part of the hollow body (bottle) whichis in the area of vision of the user (covering area and shoulder area)whereas the base area--in which the greater quantity of heat has to bedissipated and thus determines the cycle time--has a customary tooltemperature for the conventional cooling of from 7 to 15° C. For thispurpose, the tool is divided into at least one cooling zone and onetempering zone. Since the blow-molding tool is divided into at least onecooling zone and one tempering zone, on account of the cooling zone arapid cooling of the container part is possible in this region, thiszone being provided for the base area and also optionally for the neckarea in the case of bottles. As a result, although such an optimumsmooth exterior surface is not then achieved--which is also unimportantin these areas--the cycle times customary in the case of a customaryextrusion blow-molding process are nevertheless achieved in this way.

A multiple-component mixture of citric acid and salts of citric acid andcarbonic acid has been found to be advantageous as a blowing agent forforming foam with a smooth surface. The blowing agent reactsendothermically during the disintegration. In addition, carbon dioxide,which has a nucleation effect, is produced during the disintegrationreaction. The two features promote the formation of small fine gasbubbles.

A highly uniform mixing of the PE HD granulate with a grain size of from3 to 5 mm is achieved with a blowing-agent master batch on the basis ofHD PE with an average grain size of about 1 mm, and this decisivelyaffects the formation of a smooth exterior surface and a finedistribution of the gas bubbles.

An optimum mixing ratio occurs when the granulate is provided with from0.5 to 2.5% of the blowing-agent master batch.

This mixing is further assisted by the fact that in the inlet of theextruder the plastics material is heated starting with a temperaturebelow the disintegration temperature of the blowing agent with aprogressive temperature pattern towards the nozzle outlet.

In addition, the mixing is additionally optimized by a rotational speedof the extruder of from 20 to 25 r.p.m.

The production of bottle-like plastics-material bodies (bottles andtubes) by this method is particularly advantageous, since there is aconsiderable demand for them on the market.

For metering from a bottle-like plastics material it is advantageous tomake the container squeezable in a resilient manner, which is achievedin that the foamed plastics material has a 10 to 30% lower density (foamproportion) than the density of the same non-foamed plastics material.

An addition of from 5 to 20% of LD PE--preferably about 5%--with respectto the base material HD PE causes a more intensive formation and a moreuniform distribution of the gas bubbles.

For a metered discharge by squeezing against a wall of a bottle(upside-down), for example a shampoo bottle as a plastics hollow body, amaximum volume of 2.0 liters and a wall thickness of from 0.6 to 1.2 mmis provided.

EXAMPLES

A first example is described hereinafter:

A hollow body with a volume of 250 ml is produced in the extrusionblow-molding process. For this purpose, a PE HD granulate with a grainsize of from 3 to 5 mm is mixed with 1.5% of the blowing-agent masterbatch described and is melted with a suitable extruder on an extrusionblow-molding machine and is extruded as a tube. To this end, atemperature program is selected which, starting at 170° C. in the inlet,ends with an increase to 210° C. at the nozzle outlet. A masstemperature of from 205 to 215° C. is required. The rotational speeds ofthe extruder have to be determined in accordance with the diameter andlength of the extruder. Rotational speeds of from 20 to 25 r.p.m. areadvantageous. The pre-molded tube blank is gripped by the tool mold andis pressed against the tool cavity at a blow-molding pressure of from0.5 to 2.0 bar. The tool is cooled by water which is tempered to 35° C.The production process of the hollow body or bottle takes place with acycle time of about 10 s, after which the foamed bottle described isremoved from the tool. The bottle has a charging weight of 15 g and anaverage material density of 0.78 g/cm³. The exterior surface ispractically smooth and is considered to be cosmetic by the user. Thecycle time of the production process practically does not differ fromthose of a production of compact hollow bodies (bottles) in particulareven when using tempering/cooling divided with respect to the tool.

Second example:

A hollow body to be molded in a special manner (for example arectangular bottle with acute corners and edges) with a volume of 250 mlis produced in the extrusion interval blow-molding process. For thispurpose, a PE HD granulate with a grain size of from 3 to 5 mm is mixedwith 1.5% of the blowing-agent master batch described and is melted onan extrusion blow-molding machine with a suitable extruder and isextruded as a tube. For this purpose a temperature program is selectedwhich, starting at 170° C. in the inlet, ends with an increase to 210°C. at the nozzle outlet. A mass temperature of from 205 to 215° C. isrequired. The rotational speeds of the extruder have to be determined inaccordance with the diameter and length of the extruder. Rotationalspeeds of from 20 to 25 r.p.m. are advantageous. The pre-molded tubeblank is gripped by the tool mold and is blow-molded in a so-calledinterval blow-molding process, a blow-molding pressure of from 3 to 10bar--preferably from 8 to 10 bar--being provided in a first time slotand a blow-molding pressure of from 0.5 to 1.0 bar being provided in asecond time slot. A time of from 0.5 to 2.0 seconds is provided as thefirst time slot and a time of from 3 to 5 seconds is provided as thesecond time slot. The tool is cooled by water which is tempered to 35°C. The production process of the hollow body or bottle takes place witha cycle time of about 7 s, after which the foamed bottle described isremoved from the tool. The bottle has a charging weight of 15 g and anaverage material density of 0.78 g/cm³. The exterior surface ispractically smooth and is considered to be cosmetic by the user. Thecycle time of the production process practically does not differ fromthose of a production of compact hollow bodies (bottles) in particulareven when using tempering/cooling divided with respect to the tool.

What is claimed is:
 1. A method of making a hollow foamed plastic bodyfrom polyethylene in an extrusion blow-molding process, said methodcomprising the steps of:a) providing a blowing-agent master batch thatdisintegrates endothermically and provides a nucleation effect duringthe blow-molding process, said blowing-agent master batch having anaverage grain size of about 1 mm; b) making a mixture comprising apolyethylene granulate and a portion of the blowing-agent master batch,said polyethylene granulate comprising high density polyethylene; c)extruding the mixture formed in step b) with a single-screw extruder toform a pre-molded tube blank; and d) blow molding the pre-formed tubeblank at a blow-molding pressure of between 0.5 and 2.0 bar with ablow-molding tool having at least one portion of a tool surface at atool surface temperature of at least 35° C. to form the hollow foamedplastic body.
 2. The method as defined in claim 1, wherein theblow-molding agent master batch comprises a blowing agent and said highdensity polyethylene.
 3. The method as defined in claim 2, wherein theblowing agent comprises a mixture of citric acid, carbonic acid andsalts of citric acid.
 4. The method as defined in claim 1, wherein thetool surface temperature is from said 35° C. to 40° C. over said atleast one portion of the tool surface.
 5. The method as defined in claim1, wherein said blow molding tool has at least one cooling zone and atleast one tempering zone.
 6. The method as defined in claim 5, whereinsaid at least one portion of said tool surface corresponds to a coveringarea of the hollow foam plastic body.
 7. The method as defined in claim5, wherein said at least one portion of said tool surface corresponds toa covering area and a shoulder area when said hollow foamed plastic bodyis a bottle.
 8. The method as defined in claim 5, further comprisingcooling the blow-molding tool to a temperature of from 7 to 15° C. insaid at least one cooling zone and said at least one cooling zonecorresponds to a container base of the hollow foamed plastic body. 9.The method as defined in claim 1, wherein said polyethylene granulatehas a grain size of from 3 to 5 mm.
 10. The method as defined in claim1, wherein said portion of said blowing-agent master batch consists offrom 0.5 to 2.5% by weight of said mixture.
 11. The method as defined inclaim 1, wherein said polyethylene granulate comprises from 5 to 20% byweight of low density polyethylene.
 12. The method as defined in claim1, wherein said polyethylene granulate comprises 5% by weight of lowdensity polyethylene.
 13. The method as defined in claim 1, wherein saidsingle-screw extruder is operated at a rotational speed of from 20 to 25rpm.
 14. The method as defined in claim 1, wherein said single-screwextruder has an extruder inlet at a temperature below a disintegrationtemperature of the blowing agent and a progressive temperature patterntowards an extruder outlet of the single-screw extruder.
 15. The methodas defined in claim 1, wherein said hollow foamed plastic body has adensity of from 10 to 30% lower than a density of non-foam plasticmaterial having a composition that is the same as that of said hollowfoamed plastic body.
 16. The method as defined in claim 1, wherein saidhollow foamed plastic body is a bottle and said bottle has a volume ofup to 2.0 liters and a wall thickness of from 0.6 to 1.2 mm.
 17. Themethod as defined in claim 1, wherein said blow-molding pressure isbetween 1.5 and 2.0 bar.
 18. A method of making a hollow foamed plasticbody from polyethylene in an extrusion blow-molding process, said methodcomprising the steps of:a) providing a blowing-agent master batch thatdisintegrates endothermically and provides a nucleation effect duringthe blow-molding process, said blowing-agent master batch having anaverage grain size of about 1 mm; b) making a mixture comprising apolyethylene granulate and a portion of the blowing-agent master batch,said polyethylene granulate comprising high density polyethylene; c)extruding the mixture formed in step b) with a single-screw extruder toform a pre-molded tube blank; and d) blow molding the pre-formed tubeblank in an interval blow-molding process in a first time slot and in asubsequent second time slot, wherein a blow-molding pressure during theblow-molding process is between 3 and 10 bar in the first time slot andfrom 0.5 to 1.0 bar in the second time slot and with a blow-molding toolhaving at least one portion of a tool surface at a tool surfacetemperature of at least 35° C.; whereby the hollow foamed plastic bodyis formed.
 19. The method as defined in claim 18, wherein saidblow-molding pressure in said first time slot is from 8 to 10 bar. 20.The method as defined in claim 18, wherein the blowing-agent masterbatch comprises a blowing agent and said high density polyethylene. 21.The method as defined in claim 18, wherein the blowing agent comprises amixture of citric acid, carbonic acid and salts of citric acid.
 22. Themethod as defined in claim 18, wherein the tool surface temperature isfrom said 35° C. to 40° C. over said at least one portion of the toolsurface.
 23. The method as defined in claim 18, wherein said blowmolding tool has at least one cooling zone and at least one temperingzone.
 24. The method as defined in claim 23, wherein said at least oneportion of said tool surface corresponds to a covering area of thehollow foamed plastic body.
 25. The method as defined in claim 23,wherein said at least one portion of said tool surface corresponds to acovering area and a shoulder area of the hollow foamed plastic body whensaid hollow foamed plastic body is a bottle.
 26. The method as definedin claim 23, further comprising cooling the blow-molding tool to atemperature of from 7 to 15° C. in said at least one cooling zone andsaid at least one cooling zone corresponds to a container base of thehollow foamed plastic body.
 27. The method as defined in claim 18,wherein said polyethylene granulate has a grain size of from 3 to 5 mm.28. The method as defined in claim 18, wherein said portion of saidblowing-agent master batch consists of from 0.5 to 2.5% by weight ofsaid mixture.
 29. The method as defined in claim 18, wherein saidpolyethylene granulate comprises from 5 to 20% by weight of low densitypolyethylene.
 30. The method as defined in claim 18, wherein saidpolyethylene granulate comprises 5% by weight of low densitypolyethylene.
 31. The method as defined in claim 18, wherein saidsingle-screw extruder is operated at a rotational speed of from 20 to 25rpm.
 32. The method as defined in claim 18, wherein said single-screwextruder has an extruder inlet at a temperature below a disintegrationtemperature of the blowing agent and a progressive temperature patterntowards an extruder outlet of the single-screw extruder.
 33. The methodas defined in claim 18, wherein said hollow foamed plastic body has adensity of from 10 to 30% lower than a density of non-foam plasticmaterial having a composition that is the same as that of said hollowfoamed plastic body.
 34. The method as defined in claim 18, wherein saidhollow foamed plastic body is a bottle and said bottle has a volume ofup to 2.0 liters and a wall thickness of from 0.6 to 1.2 mm.
 35. Themethod as defined in claim 18, wherein said first time slot is from 0.5to 2.0 seconds in duration and said second time slot is from 3 to 5seconds in duration.